Welcome to HVAC-Talk.com, a non-DIY site and the ultimate Source for HVAC Information & Knowledge Sharing for the industry professional! Here you can join over 150,000 HVAC Professionals & enthusiasts from around the world discussing all things related to HVAC/R. You are currently viewing as a NON-REGISTERED guest which gives you limited access to view discussions

To gain full access to our forums you must register; for a free account. As a registered Guest you will be able to:

Participate in over 40 different forums and search/browse from nearly 3 million posts.

I recently bought a new house with the Lennox MPD-4035 model LP gas direct vent fireplace in the family room. The fireplace is located in a bumpout (i.e. a small framed extension that protrudes from wall-line of the rest of the house) and is vented from the top of the fireplace with a 90 degree elbow that vents to the outside.

Once the weather here in the northeast started to get cold, I noticed significant amounts of cold air coming in through the fireplace, most notably the lower grate where the gas line comes in. The walls, roof and floor of the bumpout are insulated, so I don't think that is the source of the draft. A guy from the place that installed it told me that this fireplace doesn't have a damper. Is this normal? It would seem that the cold air from outside can just enter the vent pipe and into the firebox (which is sealed, but not airtight), does this make sense??

Additionally, I built a box for a plasma TV that is about 24" above the fireplace and flush to the inside wall. There is an opening in the rear of the box that is open to the air in the bumpout (again, which is insulated). This opening is also full of frigid air when it is cold out. To solve these issues, we have stuffed fiberglass insulation in and around the opening of the TV box, in the lower grate of the fireplace as well as the top of the fireplace where the vent pipe sticks out.

Although the insulation is non-combustible, I'm confused why so much cold air is entering the space and if it is safe to operate the fireplace like this. Any help is much appreciated!!

cold air infiltration

Remove that insulation at once! It is a fire hazard blocking airflow!

Fix the house and the cold air will come in elsewhere.

First of all, insulation insulates--it does not stop cold air. Furnace filters made of insulation pass >2,500 CFM. In addition to insulation, you need Air Sealing. Walls don't leak until you put holes in them. Let's look at why cold air is coming in instead of warm air leaking out.

If you open a window, the factor that determines whether cold air infiltrates or warm air exfiltrates is pressure gradients. If there is a higher pressure outside or a lower pressure inside, cold air will seep in. Its up to you if you want to control where it comes in but the house is telling you it needs air. If you don't control the entry point, it will seek the path of least resistance, which in your case happens to be at the Fp.

Try this experiment; Open a basement window and see if you're still getting cold air in. Also, see my recent post about the smoke getting into the basement.

Hearthman, thanks for the response but I'm not sure I follow you. Don't I WANT an airtight house in the living areas?? So the past of least resistance for cold air is through my FP - wouldn't a damper help the infiltration? Should my Lennox FP have a damper?

As far as the insulation I'm using - whether it keeps hot air in or cold air out doesn't really matter to me, whatever it is doing, it seems to be working very well. And why is it a fire hazard, it is non-combustible, and nowhere in the FP installation instructions does it say you can't insulate around the firebox. Still confused - what would you do if you were me and cold air was pouring in through your gas FP? thanks again

details

I can't teach a whole course here but let's deal with separate issues:

Packing insulation or cement for that matter into a stated clearance blocks cooling airflow. You pack btw the Fp and framing and you WILL raise the temps. of that wood. Where it passed the listing tests with no more rise than 90F above ambient, it could rise enough to start fires. Listen to what I'm saying here as I'm speaking as one who investigates these things for a living. Please Don't pack insulation around fireplaces, chimneys, and vents! FYI, since fiberglass cannot pass ASTM E-136, it is considered "combustible" as far as fireplaces go.

You want the top of the house as tight as possible with designed relief down low. If you don't plan on air leaking in" such as through a dedicated makeup air system, it will find the path of least resistance, which in your case is where somebody chopped a hole in the wall, built a dogshed and installed a Fp and TV. Many holes to leak air in.

Direct vents do Not have dampers and cannot. Make sure the unit was installed in accordance with the mfrs. specs. including any gaskets around the pipe where it enters the fireplace and caulking with a high temp. silicone if allowed. You cannot modify a fireplace without voiding the warranty & listing and could cause a fire or explosion.

you usually are allowed to seal around the gas knockout with a bit of insulation or foil tape. See mfrs. instructions or call their Tech Service Dept. Have your model & serial # ready.

Thanks for your help, Hearthman. You and this thread have been incredibly helpful. I've been doing a ton of research and I think I get the whole stack effect, pressure differential, etc. that is likely causing cold air to rush in through the vent pipe, into the firebox, and ultimately into my family room.

I guess my question for you is what should I do at this point? Would tearing off the siding in the bumpout and insulating the heck out of the walls/ceiling in the bumpout raise the temperature around the vent piping enough to deter the cold air from coming in, or would you propose a different solution? Please note I am talking about insulating the walls and ceiling, which are well away from the FP and vent pipe (the width and depth of the bumpout are much larger than the FP itself). I'm confident the FP was installed correctly, there is a gasket around the pipe where it enters the FP and when I lift the front glass of the FP and put my hand under the opening for the vent pipe at the top of the firebox, that is where you feel the draft of air rushing in.

It is so frustrating to have a new house with cold air rushing into your family room!!

If there is no or very poor insulation around the inside of the doghouse then you should get that done. It should help a great deal. Direct Vent fireplaces are indoor appliances, and they should be inside the envelope of the house. It has been in the past (around here anyway), common practice to install the fireplace as if it belonged outside, nothing but plywood between the outer firebox and the cold outdoors.

BTW if you do decide to insulate in the dooghouse make sure you also put in a vapor barrier (poly) and drywall or plywood on top of that. Then seal all the seams as best you can with some hi-temp silicone sealant. This is what we do around here now and so far it has been working great.

Also since your fireplace is direct vent you should not get too much cold air that comes through the chimney into the firebox and into the house. As hearthman said the unit is sealed. There might be small gaps between pieces of sheet metal but there should not be enough air coming in (just from the chimney) to cause a draft or anything. If there is, check the gaskets around the glass and make sure they are sealing up good.

Do you have a pilot stay on all the time? If not you might want to ask a dealer if there is an upgrade you can do so that a pilot can be burning all the time. This helps keep cold air out of the firebox.

And for the record I agree with hearthman about the insulation INSIDE the fireplace..... VERY VERY bad idea. It may not say in the manual not to do it, but it also doesn't say anywhere that its a good idea. If they were to list everything NOT to do the manual would be 500 pages.

You control where the air comes in

Sure, weatherizing the doghouse will minimize the amt. of cold air infiltration at the Fp IF you can seal it well. One of the best sheathings to retain the insulation and provide an air barrier is Thermoply with the seams taped with foil tape. Don't forget under the Fp in the cantilever.

Mikey, you need to provide some air in that house--period. If you don't like where the house chooses to bring it in, it will decide for you. If the leaks around the doghouse are the last major leaks and you seal them, you could backdraft a water heater, furnace, boiler, or open Fp.

Open a basement window until the cold air quits coming in around the Fp. That's how much air you need to bring in. You may need to install a powered makeup air system. I agree w/ JTP on the pilot along with the rest of his advice.

insulation

Hearthman,

As a follow-up, just want to get a clear understanding. I'm installing a Heatilator 4842 DV Novus in my basement. The back of the unit will be 2 inches off the exterior concrete wall and sitting on a framed hearth roughly 9 inches from the floor. THe DV pipe is 90'd off the top of the unit and will go horizontally through the 10" concrete wall. It meets all clearences per manual. Currently I am going to frame around the unit and abide by all clearences in the manual. My question is when the framing is complete, can I use an R-13 Kraft faced in the framing around the fireplace? I intend on covering the insulation with a 1/2 plywood, then apply Eldorado stone to the framed out box. I have a new house and a wood burner upstairs, and I'm sure they insulated all the walls and the bump out. I don't intend to insulate anywhere inside the framed out box so I should be good. Also, since it is a basement, the framing leading up to where the fireplace sits, is a few inches off the wall. Would you recommend stuffing unfaced insulation in the 3 inches or so to prevent air infitrating the framed box where the fireplace sits. In other words, prevent drafts from behind the walls into the space? Any help is appreciated!

basement Fp

I recommend you go to http://www.buildingscience.com and get Dr. Joe's book for your climate. It will detail how best to insulate the perimeter wall of the basement rather than a bumpout you create. You can place a thermal energy break under the fireplace such as Homosote board. I would be cautious about insulation under a framed platform in a basement.

I hope you double checked the clearance from the ground to the termination before you drill a hole in the wall. A 9" core drill should do just fine. Make sure you seal it with high temp. RTV silicone. Since you're coming off the top, I recommend you get a little rise off the top, say with a 3 footer before the ell to horizontal. Unit will be happier.

When insulating, I recommend a sheet of Thermoply over the studs to hold insulation off the unit. If you do insulate the basement perimeter walls, use T-ply behind the Fp but you still must maintain clearances. Not sure where the drafts are expected to come in. Again, if you get cold drafts, install a makeup air systems to replace the drafts as an air source.

Thanks for advice. The irony of this whole thing is when I looked at your credentials, I laughed. You actually work at H and H where I purchased the TRAPK2 and 6 inch pipe. I believe Nate helped me out and we came up with an elbow off the top, then a 6 inch, to the K2. The unit will sit 2 inches in front of the 10" concrete wall (no insulation or wood behind it). The unit will also sit up off the floor 11 inches making the top of the pipe elbow around 60" probably less. That gives me ~30" from the ceiling and below the deck on the exterior. My home has a walkout basement. The vent will be many feet from any window or door or meters and ground. If you recommend the 3 foot off the top, I'll need to pick that up- I still haven't drilled the wall. However, I'll need to check my clearances because the term will be under a deck and I want more than the 12 inch minimum. I don't need the 11 inches under the unit, that was only for looks. Here is a link to a picture- http://www.pscinfogroup.com/pics/dsc02160.jpg
Thanks again for all your advice.

Cold air blowing in

I too have cold air blowing in from the gas supply line cut out of my propane fireplace. From my research this seems to be an all too common problem for many gas fireplace owners. My solution has been to shut off the gas to the fireplace and block off the cut out with insulation and duct tape. Maybe there needs to be a draft test done on outside walls before these fireplaces get installed. Cold air leakage is a major problem and should be given the same level of care as propane leakage, both stop your use and enjoyment of your gas fireplace. Feel really bad paying the yearly propane tank rental fee when my propane consumption is zero. If I need to go to the extreme of removing the fireplace to fix this problem then I might as well go the extra mile and swap the gas fireplace for an eletric one and save on the tank rental fees. Are there people out there with direct vent gas fireplaces sitting in a protruding dog house that do not have cold wind blowing in from the bottom grate of their fireplace?

carol, if the doghouse was properly insulated and sealed there would be no air coming in under there. You can however leave the gas supply turned on and jam the hole full of insulation and then foil tape over it. That should stop the air from blowing in and you can still use the fireplace.

Thank you JP for your advice and I will give it a try, but really since these doghouses are such a pandoras box should it not be part of the installation to check the airleakage from the specific area with a smoke pencil to ensure the dog house is free of Drafts. Especially in northern US and Canada.